The invention relates to a process for adjusting a processing unit, for example, an air separation unit, whereby at least one set value is varied from a starting value to a final value, the target value of a correcting value is determined based on the starting value and final value of the adjustment value, and the value of the correcting value is run up from a starting value to the target value using a parameter-dependent transfer function.
The control and adjustment of processing units is carried out automatically to an increasing extent. Thus, for automatic load adjustment of low temperature units, especially air separation units, automation programs are used that calculate the target values of the corresponding correcting values in the case of a planned increase or reduction of one or more adjustment values, for example the amount of product of oxygen or nitrogen in gaseous form and that relate the regulators that are linked to these correcting values to the target values via a slope function. To compensate for the complex dynamic behavior of these units, the correcting values are adjusted using time-delayed transfer functions, e.g., P-T1 or PD-T1 systems.
The parameters of these transfer functions still cannot be determined precisely by theoretical calculations and must therefore be determined experimentally by extensive tests during the operation of the unit. In this case, it is problematical that different parameters are also frequently necessary for different load adjustments. The search for satisfactory parameter adjustments therefore turns out to be very time-intensive. An inadequate accuracy of the parameters and thus the transfer functions results, however, in an undesirable loss of the product purities during the implementation of the load adjustment.
One object of this invention is therefore to develop a process of the type mentioned above, in which the time-consuming series of tests for determining the parameters of the transfer functions are avoided as much as possible.
Upon further study of the specification and appended claims, other objects and advantages of the invention will become apparent.
The objects are achieved according to the invention in that a control variable is measured, the value of the control variable is compared to a comparison value, and at least one parameter of the transfer function is corrected in the case of deviations of the value of the control variable from the comparison value.
In the new process according to this invention, selected processing values are measured and used as control variables to determine whether when a set value is altered, the corresponding correcting values are run up to their new target value at a rate that is too high or too low. In the case of deviations of the value of a control variable from a corresponding comparison value, the parameters of the corresponding transfer function or else several corresponding transfer functions are newly calculated and corrected during the alteration of the correcting values. In the adjustment process according to the invention, still during the adjustment process, i.e., before the corresponding adjustment and correcting values have reached their target values, the time behavior of the correcting value guide is affected.
Preferably, in the case of deviations of the value of the control variable from the comparison value to the target value, the correcting value is not altered. Thus, only the time behavior of a setpoint guide, i.e., the rate at which the value of a correcting value approaches its target value, is adjusted, but not the stationary final value, which is attained after the setpoint guide has been completed.
The detection of the control variables and their comparison to the comparison value can be carried out intermittently or continuously. An ongoing measurement of the control variables and optionally an ongoing correction of the parameters of the transfer function has proven advantageous, since boundary conditions that are imposed in this way, for example when there is a change in the load on an air separation unit, can better maintain the product purities.
In the case of a deviation of the value of the control variables from the comparison value, it is often suitable and adequate to correct the time constants of the corresponding transfer functions.
The comparison value, to which the control variable is compared, is advantageously fixed. It is also possible, however, to provide the comparison value with correction factors that depend on the previous course of the alteration of the value of the correcting value.
It is achieved by the process according to the invention that the parameter adjustments of the transfer functions no longer have to be determined exactly but rather only approximately by the series of tests. Extensive studies have shown that by calculating the experimental expense that is linked to the parameter determination and the accuracy of the correcting value guide that results therefrom, a requirement of the value of the parameter for the transfer function up to a magnitude is advantageous. The parameters are preferably specified up to a factor of 5, especially preferably up to a factor of 3. It is adequate, for example, to set a time constant at a value of between 100 and 300 seconds.
If the magnitude of the parameter values is known, simple mathematical functions are advantageously used to form an area of the measured control variables in a parameter range. In principle, this imaging function has to be selected so that an alteration of the value of the control variables has a correcting effect on the course of the correcting value guide. For this purpose, a straight equation is selected, i.e., it is postulated that there is a linear dependence of the parameters of the transfer function on the value by which the control variable deviates from the comparison value.
It is also advantageous to produce the feedback of the control variables in the parameters of the transfer function by adjusting the fuzzy logic. Since with fuzzy logic, it is typically not necessary to maintain a specific value, the transfer function can be formulated with relatively poor definition. This is reflected in a small amount of time spent for setting the parameters and for formulating the transfer function.
It has been shown that with respect to the boundary conditions that are to be maintained in the case of an alteration of a set value, it is advantageous, when the setpoint of the adjustment value is raised, to select a transfer function or a transfer function with different parameters than when the setpoint of the adjustment value is reduced.
The process according to the invention can be used preferably in all types of forward controls and perturbation variable compensations in the control and adjustment of any processing units.
The process is preferably used in the adjustment of a low-temperature air separation unit. In this connection, the amounts of one or more of the products oxygen, nitrogen or argon either in gaseous or liquid aggregate state especially preferably form the adjustment value.
As control variables, the values of liquid level, pressure, temperature flow or analysis have proven advantageous.
The process according to the invention has also especially proven worthwhile during automatic starting of processing units. To start a processing unit, it is only necessary to preset the desired setpoints in the stationary operating state, e.g., product amounts and product purities; the starting of the unit is then carried out automatically.
The new final value of an adjustment value is advantageously set based on specific events or specific control values. Thus, for example, in the control of an air separation unit, it is advantageous to observe the pressure in an oxygen product tank, the amount of product oxygen transferred to the consumer and/or its time functions. These values give indications regarding the instantaneous and expected product oxygen requirements and can therefore be used to introduce automatically either a load increase or a load reduction of the air separation unit.
The load adjustment of the processing unit preferably is not performed in a quasi-stationary manner but rather different gradients, downtimes, delays and/or suspensions are provided for individual adjustment values.
In the case of excess deviation of the value of the measured control variables from the comparison value, it has also proven worthwhile for the correcting value to avoid once a perturbation variable that compensates the deviation.
The process according to the invention provides a considerable time savings when the automation of a processing unit, especially a low-temperature air separation unit, is started up. The start-up costs of the unit can thus be significantly reduced. In addition, an increase in quality of the automation system is achieved by the new process. The required boundary conditions in the alteration of one of the adjustment values of the processing processes can be reliably maintained. Thus, for example, in a load variation of an air separation unit, maintaining the product purities is ensured.